Started 2013 May for Parisa Zareapo in Ken Burch's lab.  This apparatus measures static and dynamic (AKA differential) resistance (or conductance) of junctions with two materials, where one, both or neither may be superconducting.  A sub-Volt DC voltage with a small AF AC excitation is applied to a 4-wire sample in a cryostat.  The AC and DC voltages and currents are measured and sent back to the host (the controlling computer).  The sample is galvanically isolated from the cryostat and any ground or earth.

A DAC generates the required static voltage.  An on-board oscillator generates the AC component, the amplitude of which is controlled by a digital pot.  The DC and AC are summed in an opamp with current boosting.  This opamp output is attenuated with selectable low tempco resistors.  Four measurements by four ADCs are made: voltage and current of both the DC and AC components.  AC measurements are phase locked to the AC peak and minimum.  Although the absolute amplitude of the AC voltage and current at its peak and minimum should be the same, both are taken to reduce errors.  Similarly, the AC frequency, which is in the audio range, can be varied to reduce reactive effects of the sample, the cabling and this unit.

Voltage across the sample is with four connections: two current drives and two measurement, also known as Kelvin measurement.  The four connections are BNC.  The BNC shields are connected to the board through 100 Ohms each.  One shield must be connected to an earth ground.  Empirically, experiment with grounds on one or more shields for lowest noise.  This cannot be calculated, it must be tested with trial and error.

Two sets of four BNCs are provided.  They are identical.  While one set may be the sample under test, the other can be a standard resistor used for calibration.  Select the active set with the front panel switch.

Dial Settings